1. Field of the Invention
The present invention relates to estimating frequency offsets, and more particularly, to a method for estimating frequency offsets and related frequency offset estimation circuit through shifting a target signal from a first specific frequency band to a second specific frequency band according to a frequency shifting direction before performing a specific filtering operation and then shifting the resultant filtered signal it to a base band.
2. Description of the Prior Art
With the coming of the digital era, television broadcasting has gradually transformed from conventional analog systems into digital systems. Digital video broadcasting systems are able to overcome poor receiving quality or weak signals resulted from terrain factors by existing analog video broadcasting systems and to provide TV programs with higher quality. Present digital television broadcasting formats include the Advanced Television Systems Committee (ATSC) format in the United States, the Digital Video Broadcasting-Terrestrial (DVB-T) format in Europe, and the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) format in Japan.
Please refer to FIG. 1. FIG. 1 is a frequency spectrum diagram for an ATSC format television signal 110 according to the prior art. In the ATSC standard, a frequency band of 6 MHz is utilized for one channel, which is the same as the frequency band of the NTSC standard. As shown in FIG. 1, the ATSC format television signal 110 is in an intermediate frequency (IF) band, which has a center frequency at 6 MHz and has a frequency band of 6 MHz. The ATSC format television signal 110 further includes a pilot signal 112 located at 3.31 MHz. The ATSC format television signal 110 has a signal characteristic that the intensity of signals at higher frequencies than the pilot signal 112 is much greater than the intensity of signals at lower frequencies than the pilot signal 112, thus interference induced from signals at higher frequencies is much greater than interference caused from signals at lower frequencies.
Please refer to FIG. 2. FIG. 2A and FIG. 2B are frequency spectrum diagrams for transforming a first ATSC format television signal 210 from an IF band to a base band according to the prior art. In FIG. 2A, the first ATSC format television signal 210 is in the IF band and has a first frequency offset Δf, which is a positive value in a frequency axis (i.e., Δf>0), therefore, its center frequency is at (6+Δf) MHz. The first ATSC format television signal 210 also has a pilot signal 212 located at (3.31+Δf) MHz. The first ATSC format television signal 210 is then shifted from the IF band to the base band, as shown in FIG. 2B. In FIG. 2B, the first ATSC format television signal 210 is in the base band and the pilot signal 212 is at Δf MHz due to the positive frequency offset Δf. After frequency shifting, the first ATSC format television signal 210 is filtered by a filter 230. A slanted area 240 shown in FIG. 2B indicates the signals that interfere with the pilot signal 212 after filtering.
Please refer to FIG. 3. FIG. 3A and FIG. 3B are frequency spectrum diagrams for transforming a second ATSC format television signal 310 from an IF band to a base band according to the prior art. In FIG. 3A, the second ATSC format television signal 310 is in the IF band and has a second frequency offset (−Δf), which is a negative value in the frequency axis (i.e., (−Δf)<0), therefore, its center frequency is at (6−Δf) MHz. The second ATSC format television signal 310 also has a pilot signal 312 located at (3.31−Δf) MHz. The second ATSC format television signal 310 is then shifted from the IF band to the base band. In FIG. 3B, the second ATSC format television signal 310 is in the base band and the pilot signal 312 is at −(Δf) MHz due to the negative frequency offset (−Δf). After frequency shifting, the second ATSC format television signal 310 is filtered by the same filter 230. A slanted area 340 shown in FIG. 3B indicates the signals that interfere with the pilot signal 312 after filtering.
Please compare FIG. 2 with FIG. 3. As one can see, the slanted area 240 shown in FIG. 2B is much smaller than the slanted area 340 shown in FIG. 3B. Therefore, a demodulator for the ATSC format television signals has a tolerance of the positive frequency offset higher than a tolerance of the negative frequency offset. If the negative frequency offset is greater than a specific value (such as 100 KHz), this will easily result in wrong frequency offset estimation and cause synchronization failure. Therefore, there still needs efforts for improving the negative frequency offset estimation.